MFX (or MIDI FX) is a part of the DirectX music
specification that allows for the processing of MIDI
data. All recent Cakewalk products (SONAR, Home Studio,
etc) can use MFX plugins, and recent Cubase (SX/SL)
offerings can use them through a wrapper provided by
Steinberg.

EmGate is a MIDI-controlled channel mixer gate. It
can be triggered by MIDI Note messages for gated
behavior, or tweaked gradually using the Mod Wheel as a
fader. (No big
explanation here: just download it and read the text file!)